Crazy idea for gear reduction

can someone measure the shaft size on the pedal's shaft on left side?
If we could use the left facing shaft on the motor link up by chain straight down to pedals shaft ( remove the pedals ) and that could potentionaly give us ie 10 18 21 speed gearing that is allredy on the bike...I imagine you have to be careful on shifting while the engine is under power... could this work guys???

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For clarity, we're not talking about a motor assisted bike here. The result would (I believe) be illegal on the roads of any state. You are basically talking about using the bottom bracket as a jack shaft. Using the bikes gearing: It would only be using the multiple gears on the rear wheel, so we're talking about 5 - 10 gears, not 10 to 21. Much of that gearing would not result in an increase in speed. It could be used for ultra low end. But there are limits to how much you might want to gear down. For top end there are limits dictated by aerodynamics. There are places on the web where this can be easily researched. I haven't tried using the bike gears myself, but I've heard rumors that motors eat derailers.
For moving the power along the left side of the bike, southpaw hubs and freewheels exist, and could be used.

I believe what you're talking about can and has been done. Check the Staton website and check the two wheel chain drive section under Giant Stiletto.
On this bike they use a jack shaft and freewheel sprockets to run the engine power to the dérailleur. The freewheels allow you to run the engine and the petals independent of each other.

the problem I am runing in to that the setup that I am after is low reving engine 4 stroke OHV ( low nosie level ) 4 hp + so I can cruise at 2500 rpm or so....the setup does not have built in gear reduction so I have to figure out how to do the setup that somewhat looks clean and reliable...as for using pedals function....as long as they work ( dont have to work well to use 100% of time they will leave you alone...like moped I had was considered motor asist bicycle...didnt have to register it or nothing just give'er

I found that the best way to do this was to build a frame from scratch and use a jack shaft.
I have used a shimano 8 speed hub which has no problem with the power, as for speed I have got up to 80kmh. cruse around town at your requested 2500rpm no problem. Shifting with this set up is great and smooth, I dont think that the derailar set up would cut it.

I made my own jack shaft using a cassette hub. This way you can play around with ratios that suit your needs.
You end up with a fixed sprocket to the motor and rear wheel. and the freewhell off the casstte to the pedels
workes well and really needs a pull start motor.